The present invention relates to a process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine (I), in which a liquid stream comprising formic acid and tertiary amine (I) in a molar ratio of from 0.5 to 5 is produced by combining tertiary amine (I) and a formic acid source, and formic acid is removed by distillation at a temperature at the bottom of from 100 to 300° C. and a pressure of from 30 to 3000 hPa abs from the resulting liquid stream in a distillation apparatus, where the bottom output from the distillation apparatus is separated into two liquid phases and the upper liquid phase is recirculated to the formic acid source and the lower liquid phase is recirculated to the removal of the secondary components and/or to the distillation apparatus.
Formic acid is an important and versatile product. It is used, for example, for acidification in the production of animal feeds, as preservative, as disinfectant, as assistant in the textile and leather industry, as a mixture with its salts for deicing aircraft and runways and also as synthetic building block in the chemical industry.
The most widespread process at present for preparing formic acid is the hydrolysis of methyl formate which can be obtained, for example, from methanol and carbon monoxide. The aqueous formic acid obtained by hydrolysis is subsequently concentrated, for example using an extraction auxiliary such as a dialkylformamide (DE 25 45 658 A1).
In addition, obtaining formic acid by thermal dissociation of compounds of formic acid and a tertiary nitrogen base is also known. These compounds are generally acidic ammonium formates of tertiary nitrogen bases, in which the formic acid has reacted beyond the stage of classical salt formation with the tertiary nitrogen bases to form stable addition compounds bridged by hydrogen bonds. The addition compounds of formic acid and tertiary nitrogen bases can be formed by combining the tertiary nitrogen base and a formic acid source. Thus, for example, WO 2006/021,411 discloses the preparation of such addition compounds in general by (i) direct reaction of the tertiary nitrogen base with formic acid, (ii) by transition metal-catalyzed hydrogenation of carbon dioxide to formic acid in the presence of the tertiary nitrogen base, (iii) by reaction of methyl formate with water and subsequent extraction of the resulting formic acid by means of the tertiary nitrogen base and (iv) by reaction of methyl formate with water in the presence of the tertiary nitrogen base.
The general advantages of using addition compounds of formic acid and tertiary nitrogen bases for obtaining formic acid are that the addition compounds firstly bind the formic acid strongly enough to withdraw the formic acid as free formic acid from the medium, for example the reaction medium, in which the formic acid has been formed by chemical synthesis or, for example, from a dilute formic acid solution and thereby allow the formic acid to be separated off more readily in the form of its addition compounds, but are weak enough for the formic acid subsequently to be able to be released again from the addition compounds by thermal dissociation in order to obtain it in concentrated and purified free form.
EP 0 001 432 A discloses a process for obtaining formic acid by hydrolysis of methyl formate in the presence of a tertiary amine, in particular an alkylimidazole, to form addition compounds of formic acid and the tertiary amine. The hydrolysis mixture obtained, which comprises unreacted methyl formate, water, methanol, addition compounds and tertiary amine, is freed of the low boilers methyl formate and methanol in a first distillation column. In a second column, the remaining bottom product is dewatered. The dewatered bottom product from the second column, which still comprises addition compounds and tertiary amine, is then fed to a third column and in this the addition compounds are thermally dissociated into formic acid and tertiary amine. The formic acid liberated is removed as overhead product. The tertiary amine collects in the liquid phase and is recirculated to the hydrolysis.
DE 34 28 319 A discloses a process for obtaining formic acid by hydrolysis of methyl formate. The hydrolysis mixture obtained, which comprises unreacted methyl formate, water, methanol and formic acid, is freed of the low boilers methyl formate and methanol in a first distillation column. The aqueous formic acid obtained at the bottom is subsequently extracted with a relatively high-boiling amine, in particular a relatively long-chain, hydrophobic C6-C14-trialkylamine, in the presence of an additional hydrophobic solvent, in particular an aliphatic, cycloaliphatic or aromatic hydrocarbon, and thereby converted into an aqueous addition compound of formic acid and the amine. This is dewatered in a second distillation column. The dewatered addition compound obtained at the bottom is then, according to the teaching of DE 34 28 319 A, fed to the uppermost plate of a distillation column (in FIG. 1 denoted as “K4”) and thermally dissociated. The hydrophobic solvent is present both in the overhead stream and the bottoms from the column. The gaseous overhead stream comprises mainly the formic acid liberated together with the hydrophobic solvent. This stream is liquefied again in the condenser. This results in formation of two phases, namely a polar formic acid phase and a hydrophobic solvent phase. The formic acid phase is discharged as product and the solvent phase is returned as runback to the column. Due to the presence of the hydrophobic solvent, complete dissociation of the adduct, which according to the teaching of the DE first publication occurs without decomposition of formic acid, can be achieved. The (virtually) formic acid-free bottoms comprise the hydrophobic amine and the hydrophobic solvent. This is recirculated to the extraction stage.
EP 0 181 078 A and EP 0 126 524 A describe processes for obtaining formic acid by hydrogenation of carbon dioxide in the presence of a transition metal catalyst and a tertiary amine such as a C1-C10-trialkylamine to form an addition compound of formic acid and the tertiary amine, work-up of the hydrogenation output to separate off the catalyst and the low boilers, replacement of the amine base by a weaker, higher-boiling tertiary amine, in particular by an alkylimidazole, with splitting-off of the first tertiary amine and subsequent thermal dissociation of the newly formed addition compound in a distillation column. According to EP 0 181 078 A, FIG. 1, the stream comprising formic acid and amine is for this purpose fed into the middle region of the column “30”. The formic acid liberated in the thermal dissociation is removed as overhead product. The weaker, higher-boiling tertiary amine collects at the bottom and is recirculated to the stage of base exchange.
WO 2008/116,799 discloses a process for obtaining formic acid by hydrogenation of carbon dioxide in the presence of a transition metal catalyst, a high-boiling polar solvent such as an alcohol, ether, sulfolane, dimethyl sulfoxide or amide and a polar amine bearing at least one hydroxyl group to form an addition compound of formic acid and the amine. According to the teaching of WO 2008/116,799, the hydrogenation output can be fed directly to a distillation apparatus for thermal dissociation of the addition compound. This can comprise a distillation column and, if short residence times are desired, also a thin film evaporator or falling film evaporator. The formic acid liberated is removed as overhead product. The polar amine and the polar solvent and any catalyst which has not been separated off collect at the bottom and can be recirculated to the hydrogenation stage.
WO 2006/021,411 describes a process for obtaining formic acid by thermal dissociation of an addition compound of formic acid and a tertiary amine (quaternary ammonium formate), in which the tertiary amine has a boiling point of from 105 to 175° C. Alkylpyridines are mentioned as preferred tertiary amines. The specific boiling range of the tertiary amines increases the color stability of the formic acid obtained. The addition compound to be used can in general be obtained from the tertiary amine and a formic acid source. The output from the adduct synthesis is advantageously firstly freed of volatile constituents and then fed to the thermal dissociation. The thermal dissociation is carried out as usual in a distillation column, with the stream comprising formic acid and amine being fed as per FIG. 1 of WO 2006/021,411 into the middle region of the column (C). The formic acid liberated is removed as overhead product. The tertiary amine which may still comprise residues of formic acid collects in the liquid phase and can be recirculated to the formic acid source.
EP 0 563 831 A reports an improved process for the thermal dissociation of an addition compound of formic acid and a tertiary amine (quaternary ammonium formate) to give formic acid. The addition compound to be used can in general be obtained from the tertiary amine and a formic acid source. The output from the synthesis is advantageously firstly freed of volatile constituents and then fed into the middle of a distillation column for thermal dissociation. The improvement comprises essentially carrying out the thermal dissociation of the addition compound in the presence of a secondary formamide which increases the color stability of the formic acid obtained. The formic acid liberated is removed as overhead product. The tertiary amine and the secondary formamide collect in the liquid phase and can be recirculated to the formic acid source.
PCT/EP2011/060770 teaches a process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine (I), in which combining tertiary amine (I) and a formic acid source produces a liquid stream comprising formic acid and a tertiary amine (I) in a molar ratio of from 0.5 to 5, from 10 to 100% by weight of the secondary components comprised therein are separated off and formic acid is removed by distillation from the resulting liquid stream in a distillation apparatus at a temperature at the bottom of from 100 to 300° C. and a pressure of from 30 to 3000 hPa, and the bottom output from the distillation apparatus is separated into two liquid phases of which the upper liquid phase is enriched in tertiary amine (I) and is recirculated to the formic acid source and the lower liquid phase is enriched in formic acid and is recirculated to removal of the secondary components and/or to the distillation apparatus.
It is an object of the present invention to discover an improved process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine, which process has advantages over the prior art and is able to give formic acid in high yield and high concentration. In particular, the improved process should also function stably over long operating times and produce formic acid in constant high purity. The process should naturally be able to be carried out very simply and with a very low energy consumption.
We have surprisingly found a process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine (I) which at a pressure of 1013 hPa abs has a boiling point which is at least 5° C. higher than that of formic acid, in which    (a) a liquid stream comprising formic acid and tertiary amine (I) and having a molar ratio of formic acid to tertiary amine (I) of from 0.5 to 5 is produced by combining tertiary amine (I) and a formic acid source;    (b) from 10 to 100% by weight of the secondary components comprised therein are separated off from the liquid stream obtained from step (a);    (c) formic acid is removed by distillation from the liquid stream comprising formic acid and tertiary amine (I) obtained from step (b) in a distillation apparatus at a temperature at the bottom of from 100 to 300° C. and a pressure of from 30 to 3000 hPa abs, where the tertiary amine (I) to be used in step (a) and the degree of separation in the abovementioned distillation apparatus are selected so that two liquid phases are formed in the bottom output;    (d) the bottom output from the distillation apparatus mentioned in step (c) is separated into two liquid phases, where the upper liquid phase has a molar ratio of formic acid to tertiary amine (I) of from 0 to 0.5 and the lower liquid phase has a molar ratio of formic acid to tertiary amine (I) of from 0.5 to 4;    (e) the upper liquid phase from the phase separation in step (d) is recirculated to step (a); and    (f) the lower liquid phase from the phase separation in step (d) is recirculated to step (b) and/or (c),wherein    (g) low boilers which at a pressure of 1013 hPa abs have a boiling point which is at least 5° C. lower than that of the tertiary amine (I) are separated off by distillation from the upper liquid phase from the phase separation in step (d) in a distillation apparatus at a temperature at the bottom of from 100 to 300° C. and a pressure of from 1 to 1000 hPa abs and the stream depleted in low boilers is recirculated to one of the abovementioned steps (a) to (f).